The present invention relates to a single-chip SONET physical layer device and, more particularly, to a device which supports the ATM cell protocol and the PPP protocol, while interfacing to common existing components.
A Synchronous Optical NETwork (SONET) is an optical signaling format defined for North America. The international Synchronous Digital Hierarchy (SDH) is closely related to the SONET signaling format, and the term "SONET" will be used in the following specification to refer to both SONET and SDH. A SONET STS-1 data frame consists of 9 rows of 90 octets each, for a total of 810 octets. An octet is a unit of information consisting of 8 digital bits. The term byte is typically used interchangeably with the term octet. The SONET STS-1 data frame is divided into two fields, a section and line overhead field and a synchronous payload envelope (SPE) field. The section and line overhead field includes 27 octets. The SPE field includes 783 octets of payload and 9 octets of path overhead.
Current data packet transmissions over SONET communication links are typically based upon the Asynchronous Transfer Mode (ATM) cell protocol in which ATM cells are placed into a SONET SPE and transmitted over a SONET communications link. Alternative modes of transmission are being defined using the point-to-point protocol (PPP). A standard ATM cell consists of 53 octets, which includes a 5-octet header field and a 48-octet payload field. The ATM cells "flow" along a physical medium, such as an OC-3 optical fiber or a similar carrier. The ATM cells are used for carrying data, such as data files, between two networked PCs or voice data (voice conversations) along multiple telephone lines between two large telephone distribution points. Since data files and digitized telephone conversations are typically larger than the 48 octets that would fit into the ATM cell payload field, it is necessary to repackage the data in some form. The device that performs the repackaging is referred to as an ATM adaption layer (AAL). The AAL segments the data, which is typically referred to as a protocol data unit (PDU), into 48 octet-sized pieces that fit into the ATM cell payload fields. Different types of AALs exist to segment PDUs in different ways, with each type being appropriate for a particular application. For example, AAL1 is typically used for voice traffic while AAL5 is typically used for file transfer and Ethernet traffic.
The PPP protocol is a High Level Data Link Controller (HDLC) based protocol. The PPP protocol is a popular and practical protocol for moving data traffic between two computer systems The most typical use of the PPP protocol is for computer dial-up communication with an Internet Service Provider (ISP) over telephone lines. With a Windows.RTM. 95 based personal computer, this dial-up connection is typically a PPP connection, since the PPP protocol support is built into Windows.RTM. 95. The PPP protocol consists of a data framing specification and a management protocol. PPP can be implemented with 8-bit asynchronous links, bit-synchronous links or octet-synchronous links. With octet-synchronous links, all operations are based on complete octets and individual bits are not operated on, nor inserted. A PPP data frame of this type includes several fields, including start flag, address, control, protocol, payload, error correction and end flag fields. The payload field typically consists of 0 to 1,500 octets.
Several transmission methodologies exist for transmitting ATM and PPP over SONET communication links. These methodologies include transmitting ATM cells in SONET SPEs, transmitting PPP frames in ATM cells in SONET SPEs, and transmitting PPP frames directly in SONET SPES. Transmitting ATM cells in SONET SPEs is a current market norm. The data unit or frame to be transmitted is segmented by an ATM adaption layer device into multiple ATM cells and then transmitted to a SONET physical layer device. The SONET physical layer device receives the ATM cells and places them into the SONET SPE for transmission over a SONET communications link. A typical component for receiving ATM cells and placing the cells in SONET SPEs is the S/UNI Lite available from PMC-SIERRA, Inc.
Transmitting PPP frames in ATM cells in SONET SPEs is a non-standard transmission mechanism. The data unit to be transmitted is processed into a PPP frame using a form of an HDLC processor in the octet-synchronous mode. The resulting PPP frame is passed to an ATM adaption layer device which segments the PPP frame and places the segments into ATM cells, either in a standard manner or in a non-standard manner by placing the segments into both the cell payload field and the cell header field (excluding an HEC byte). The later is a more typical approach as it increases utilization of the link. The resulting cells are passed to a SONET physical layer device and placed into a SONET SPE. All header processing is disabled. The S/UNI part can operate in this manner using a diagnostic mode.
Finally, transmitting PPP frames directly in SONET SPEs was proposed in Internet Engineering Task Force (IETF) Request For Comment (RFC) 1619, entitled "PPP over SONET/SDH" by W. Simpson. The data frame to be transmitted is processed into a PPP frame by an HDLC based controller and then loaded directly into a SONET SPE. This methodology increases the utilization of the link in two ways. First, the ATM cell header overhead is eliminated. Second, the overhead due to unused ATM cell space is eliminated. Implementation of this methodology currently requires multiple numbers of components.
The PPP over SONET transmission approach provides dramatic improvements in utilization of the bandwidth of the physical link. These improvements can go from approximately 60%-90% of bandwidth utilization for actual data transfer. This represents a 50% improvement of bandwidth utilization. However, multiple parts are required to implement PPP over SONET, which increases board space, increases cost and requires a non-standard implementation due to a lack of conforming parts. In addition, there is a lack of ability to readily switch between standard and non-standard transmission modes.